All over the country, cities with old, often crumbling, sewer systems are turning to “green infrastructure” to help manage stormwater, reduce flooding and sewage overflows, and handle the impacts of climate change. But how well these systems will work is still unknown.
In Philadelphia, they’re spending more than a billion dollars on green infrastructure, including planting more than 700 trees to soak up stormwater. In Cincinnati, they’re bio-engineering a stream to stop pollution from getting into the Ohio River. And in the next year, Pittsburgh is planning a dozen projects on the East End, including installing special pavement that soaks in water so it doesn’t rush into the sewer system. The cost? Ten million dollars.
That’s big bucks. But it’s still a lot less than laying new sewer pipes and building new water treatment systems. Still, one big question remains: Will green infrastructure actually work as well as traditional gray infrastructure?
LISTEN: Pittsburgh’s Big Plans for Green Infrastructure
Soil scientist John Buck, who works with Civil & Environmental Consultants, Inc., is among the researchers trying to figure this out. At Chatham University’s Eden Hall Campus north of Pittsburgh, Buck is digging holes with a giant fence post digger. He’s not actually building a fence. He’s putting metal sensors into the holes to measure how water moves through the layers of soil.
“The sensors are going to be laid in three inches, 12 inches and 20 inches below ground surface,” Buck says. “And from there, we can see how fast it’s moving through the soil.”
Buck and his students bury the sensors, leaving long wires sticking out of the ground. They plug the wires into a sensor hub, which kind of looks like a circuit breaker box zip-tied to a wooden pole. It also has a wireless transmitter, so Buck can check the soil readings from the box at any time, right from his phone.
“It’s telling me that the most recent reading was today at 4:10, and it tells me the status of every sensor,” Buck says.
And that will tell policymakers whether rain gardens, green roofs and porous pavement can help solve their cities’ water problems.
“At a bare minimum, you’re going to want to measure how much water is coming into your green infrastructure system and how much is going out of it,” Buck says. “And the difference between those two things is what your effectiveness is.”
Buck helps monitor several projects around the region. One rain garden in Pittsburgh’s East Liberty neighborhood captured almost all—99 percent—of stormwater runoff last year. This past August, it even handled what experts called a “once-in-167-year-storm,” when the garden soaked up nearly three inches of rain in just two hours.
That kind of effectiveness is important in regions like Pittsburgh, where untreated sewage regularly drains into the rivers after it rains. The Environmental Protection Agency is pushing sewer districts to reduce overflows into the waterways. And in Allegheny County and elsewhere, the agency is allowing the use of green infrastructure to help solve the problem.
There are already many small green infrastructure projects in Pittsburgh and other cities, but researchers are still figuring out the best ways to test how well they work.
“The more monitoring data we have, the better we’ll be able to determine how successful it is in capturing stormwater runoff,” says Beth Dutton of Three Rivers Wet Weather.
Dutton’s organization maintains a map of green infrastructure projects—large and small— across the region. Right now, her map has roughly 200 little colored dots—each signifying a rain garden, bioswale or green roof. She’s been cataloging these at schools, hospitals and apartment buildings—adding them to the map when she learns about a new one from a colleague or reading about them in the newspaper.
“If you click on the blue square, you’ll see the project name,” she says. “I was also able to provide the project size, the volume, how much roof-water runoff is being collected. And wherever possible, we went around and snapped pictures.”
But Dutton says very few sites actually are being monitored.
“It’s so expensive to set up the monitoring, and it has to be done over a significant period of time because you want to capture the different seasons and a couple of good storms,” she says.
As cities spend millions—and in some cases, billions—on these projects, it’s the job of people like James Stitt to make sure they’re working. Stitt is with the Pittsburgh Water and Sewer Authority, and to start monitoring, he says they first need to know how much stormwater is getting into the system now.
“We’re putting some monitors in the sewer pipes in the ground and getting some calculations on what those annual volumes are in that area before we do anything,” he says. “Then, we’re going to continue to monitor that for a year after construction. Every project we do, we want to learn how to do this more correctly and more effectively.”
Pittsburgh’s multi-year “Green First” plan is expected to keep one billion gallons out of the combined sewer system per year. That’s about 10 percent of the problem in Allegheny County, given current conditions. But as the climate changes, experts expect bigger storms and increased rainfall in the region. Still, Stitt thinks the green projects will fare well given those challenges.
“One of the great things about green infrastructure is it’s much more adaptive in those situations than a tunnel project or a large pipe project. That’s a stagnant solution. It’s always going to be that size. The green infrastructure—it’s a little more flexible,” he says.
Stitt says cities with more experience have found that as rain gardens and bioswales mature, they become more effective. That adaptability is one reason the EPA is pushing for more green projects to help flood-prone regions manage extreme weather.
Robert Traver, director of Villanova University’s Urban Stormwater Partnership, monitors green infrastructure with the EPA in Philadelphia. He says their projects are performing better than expected, even on very wet days. He also says a change in rainfall might call for different types of plants in a rain garden or constructed wetland.
“These systems are more resilient and are actually going to be much better for climate change than some of the past practices we’ve used,” he says.
Traver says as storms get more intense, green infrastructure won’t be able to eliminate all the overflows. So cities will likely need reliable systems that combine both gray and green.